Experimental Rock Mechanics Research Articles

Large scale analogue experiments in rock mechanics : new experimental facility, 3D-printed material and insertion of force transducers for stresses measurement.

Abstract The aim of this paper is to present a new experimental device that was designed to perform analogue rock mechanics experiments with 3D-printed material. The device consists in the creation of two rectangular excavations (mining tunnels) in slices of 3D sand printed material. This is a true-triaxial test device that allows us to study the mechanical behaviour of reduced scale models of underground structures. The experimental protocol entails the horizontal loading of the specimen, followed by the excavation of two galleries while the specimen is maintained under a constant load. The results of the test carried out in DiMiTri show a generally logical evolution of the sensors with an increase in vertical stresses induced by the presence of excavations. This paper discusses the upcoming challenges in terms of instrumentation that will be encountered, together with the new opportunities for future research.

Experimental and Numerical Investigations of Low-Permeability Sandstone Under Water Injection–Induced Dilation in West Oilfield, South China Sea

With the development of offshore oil fields, the reservoirs of X oilfield in the west of the South China Sea are poor in physical property, serious in pollution, and increasingly prominent in interlayer contradictions. Water injection dilation technology has strongly affected the development of loose sandstone reservoirs. To explore whether this technology applies to the low-permeability sandstone of X oilfield in the west of the South China Sea and the dilation effect and radius of water injection dilation technology on the target reservoir, low confining pressure rock mechanics experiments and numerical simulation of water injection in this reservoir section are carried out. The triaxial shear experiment of low confining pressure shows that the target reservoir sandstone with low-permeability can have a shear strength of 45 MPa when the effective confining pressure is 0.5 MPa, and the target reservoir core can have dilatancy. When the axial strain is 2.5%, the core dilatancy is 1%, and the permeability changes by 1.17 times. It was found that the core volume dilation was obviously under low effective confining pressure, and the permeability is 2 orders higher than in the initial condition. The numerical simulation of the target reservoir shows that the bottom-hole pressure reaches 47.12 MPa at the end of water injection in typical wells. The reservoir was deformed to different degrees around the well, and the top layer was raised by 5.58 mm. This paper characterizes the rock expansion potential and expansion flow capacity of low-permeability sandstone reservoirs from multiple perspectives and establishes a three-dimensional, full-size wellbore formation crustal stress strict matching geological model for offshore expansion wells. We have provided theoretical guidance for on-site construction.

Experimental study on physico-mechanical responses and energy characteristics of granite under high temperature and hydro-mechanical coupling

Thermal and hydro-mechanical coupling effects on granite's physico-mechanical responses and energy characteristics can influence its carrying capacity, hydraulic and heat transfer performance. The evolution of these properties is crucial for geothermal reservoir stability assessment and the optimization of deep geothermal energy development techniques. Hence, a variety of physical tests and rock mechanics experiments were conducted. Key findings include: (1) As temperature rises, granite undergoes thermal expansion and structural integrity degradation. The peak strength σp, elastic modulus E, total energy density U, and elastic energy density Ue initially rise at 25 °C–150 °C and subsequently decrease above 150 °C, while the proportion of dissipated energy ηd is opposite. Granite also initially experiences hardening, then turns to brittle-ductile transition. (2) The crystallinities of quartz, albite, and orthoclase demonstrate substantial deterioration beyond 450 °C. (3) 150 °C and 450 °C are regarded as the temperature thresholds for mechanical properties. (4) As confining pressure rises, granite experiences hardening, with σp, E, U, and Ue increasing, and ηd decreasing. (5) Pore water pressure increases ηd and decreases σp, E, U, and Ue, and its effect on the mechanical responses is pronounced when it reaches 80 % of confining pressure or exceeds 450 °C.

Study on Failure Characteristics and Acoustic Emission Laws of Rock-like Specimens under Uniaxial Compression

Complex underground conditions make it challenging to conduct extensive coring, and it is difficult for laboratories to carry out a large number of rock mechanics experiments due to the limited number of cores. Rock-like specimens are commonly used in the laboratory to replace coal-rock specimens. In this paper, rock-like specimens with different proportions are produced to study the mechanical properties, failure characteristics, and acoustic emission laws of rock-like specimens under uniaxial compression. The results show that when the rock-like specimen does not contain gypsum, the stress of the specimen decreases rapidly after reaching the peak stress, which is similar to the mechanical properties of hard brittle rock. When the rock-like specimen contains gypsum, the stress of the specimen decreases slowly after reaching the compressive limit, and the failure of the specimen is gentle, which is similar to the mechanical properties of soft rock. When the specimen lacks gypsum, the strength of the rock-like specimen is larger, and the strength of the specimen is positively correlated with the proportion of cement. When the specimen contains gypsum, the strength of the rock-like specimen decreases sharply, and the strength of the rock-like specimen is negatively correlated with the proportion of gypsum. The maximum acoustic emission ringing count increases with higher cement content but decreases with increased gypsum content. The cumulative count change of acoustic emissions approximately underwent four stages, aligning well with the four stages of uniaxial compression failure observed in typical rock. The research results have important reference value for the selection of rock-like materials to replace the original rock materials for laboratory research.

Study on Wellbore Stability of Shale Oil Drilling

Abstract With conventional oil and gas development entering the middle and late stage, unconventional oil and gas development is the focus of resource replacement and stable production. The South China Sea is rich in shale oil resources and has great development potential. However, the experience of shale oil exploitation in the South China Sea is not enough. The performance of oil shale rock in the target block is unclear, the anisotropy caused by bedding has great influence, the formation has abnormal high pressure, and various factors affect the wellbore stability. In the process of drilling, complex conditions occur frequently, which seriously limits the efficiency of exploration and development. Therefore, this paper studies the wellbore stability of shale oil drilling in the South China Sea, carries out rock mechanics experiments, analyzes the strength of oil shale rock, and studies the influence of oil-based drilling fluid and anisotropy caused by bedding on rock strength. Combined with logging data, the change law of formation caving pressure is analyzed and formation pressure is predicted. The research shows that the oil shale rock in this area has low strength, the core is hard and brittle, and the core is anisotropic under the influence of bedding. The strength weakening law of oil shale soaked in oil-based drilling fluid is studied, and the formation collapse pressure variation law is further obtained. The fracture pressure, caving pressure and pore pressure of the proposed drilling are predicted, which provides technical support for the safe and efficient drilling of shale oil drilling in the South China Sea.

Research on the Movement of Overlying Strata in Shallow Coal Seams with High Mining Heights and Ultralong Working Faces

To study the roof movement and ground pressure evolution characteristics of an ultralong working face in a shallow coal seam with a high mining height, the Shangwan Coal Mine in the Shendong mining area was used as the research background, and the physical and mechanical parameters of the surrounding rock were determined through rock mechanics experiments. A physical simulation model was built considering the 7 m mining height of the 12301 fully mechanized working face of the Shangwan Coal Mine to simulate and study the evolutions of the movement, fracture and collapse of the coal seam, direct roof, and basic roof and overlying strata during the mining process. The mechanical characteristics of the support, mechanism of roof collapse, and changes in the working resistance of the support were analysed and simulated. The research results indicate that when mining at a height of 7 m, the direct roof and basic roof strata collapse in layers; the basic roof strata collapse backwards, the rock block arrangement is more irregular, and the range of the basic roof that can form structural rock layers extends higher. After the basic roof rock fractures, it cannot form a masonry beam structure and can only form a cantilever beam structure. The periodic fracture of the cantilever beam causes periodic pressure on the working face. These research results are of great significance for planning the further mining of shallow coal seams with high mining heights and ultralong working faces in the Shendong mining area, as well as for improving the control of overlying strata.

A geometric variation method to extract rock strength parameters via uniaxial compression testing

Numerical modelling in rock engineering and mechanics often requires inputting multiple sets of strength parameters, such as the commonly encountered tensile strength, cohesion, and friction angle. Usually, obtaining these parameters requires performing different types of rock mechanics experiments on certain rock specimens, such as tension tests and triaxial tests, which are time-consuming and labor-intensive. Uniaxial compression testing is simpler and requires less demanding equipment. This paper proposes a solution that relies solely on uniaxial compression testing to obtain multiple sets of rock strength parameters. The basic idea is to treat the process of acquiring multiple parameters as a problem of solving a set of mathematical equations. The uniaxial tests with different geometric specimens provide known quantities and the corresponding equations that link these known quantities (e.g., peak loads) with unknown quantities (e.g., strength parameters). We established three-dimensional numerical tests to represent the relationship between input strength parameters and the peak load. By transforming it into an optimization problem, numerical modelling is embedded into the objective function of the optimization algorithm, thereby achieving the automatic extraction of strength parameters. This method is verified through uniaxial compression tests using red sandstone specimens with various geometries. The results show that by changing the geometry of the rock specimens, multiple sets of strength parameters can be obtained. These parameters are successfully used to predict (extrapolate) peak loads of other geometric specimens. This method holds promise for achieving the automated extraction of material parameters for the numerical modelling of rock.

Multiscale fracture, physical and mechanical properties of stromboli volcano (Italy) edifice

The physical, mechanical and fracture properties at Stromboli volcano have been integrated at multiple scales to understand whether the interplay between a presumed NE/SW rift zone and the Sciara del Fuoco (SDF) depression has resulted in a zone of weakness able to promote fracturing prone to flank instability. Multiscale fracture quantification by imaging via FracPaQ toolbox both fractures and sample scale fractures has been integrated with rock physics and rock mechanics experiments on cm-scale samples belonging to the Paleostromboli, Vancori, Neostromboli, Pizzo and Present Deposit volcanic cycles that have been taken from within and outside the rift zone. The structural changes to the edifice have been quantitively assessed by mapping at different scale fracture properties such density and orientation within and outside the rift zone allowing to identify the potential damaged zones that could reduce the edifice strength.Results indicate that basalt textures, microfracture density, porosity, chemical zoning and preferential alignments, despite lithologically dependent, can be related to the NE/SW zone of weakness at the regional scale and to collapsed volumes that have been subject to continuous intrusive activity. Numerical inversion models have been performed to cross correlate fracture density in the basalts at multiple scales.A link between microfracture density and seismic velocities has been also established via numerical modelling, allowing to interpret in terms of degree of fracturing the results of seismic tomographies at the field scale, providing a novel method to image crack damage evolution within the inner structure of the volcano edifice.

The Permian Cornubian granite batholith, SW England; Part 2: Gravity anomalies, structure, and state of isostasy

A new compilation of Bouguer gravity anomaly data has been used, together with forward and inverse modeling, to reappraise the structure, volume, and state of isostasy of the Cornubian batholith of SW England. We show the individual plutons that comprise the batholith are, on average, ∼10−11 km thick, are outward-sloping in their upper 2−3 km, and are underlain by roots which protrude downward into the middle crust. The batholith volume is estimated within the range of 76,367 ± 17,286 km3, significantly larger than previous estimates. Granite outcrops correlate with elevated topography, and mass balance calculations show that the mass deficiency of the granites relative to their host metasedimentary rocks is approximately equal to the mass excess of the topography relative to air. The existence of roots beneath individual plutons is in general agreement with predictions of an Airy model of isostasy and a depth of compensation that is within the crust rather than at the Moho. In addition, a middle crust compensation depth is compatible with the origin of the granites by heating and melting of metasedimentary rocks and with data from experimental rock mechanics which suggest that at the melting temperature and pressure of granite formation, deformation is likely to be plastic and controlled by glide along dislocations. During pluton emplacement the middle crust would, therefore, have acted as a mechanically weak layer, effectively decoupling the topography from any support it might otherwise have received from the lower crust and/or upper mantle.

Mechanical properties of minerals in lunar and HED meteorites from nanoindentation testing: Implications for space mining

AbstractThis study analyzes the mechanical and elemental properties of lunar meteorites DHOFAR 1084, JAH 838, NWA 11444, and HED meteorite NWA 6013. Utilizing microscale rock mechanics experiments, that is, nanoindentation testing, this research reveals significant heterogeneity in both mechanical and elemental attributes across the mineral samples. Olivines, pyroxene, feldspar, and spinel demonstrate similar compositional and mechanical characteristics. Conversely, other silicate and oxide minerals display variations in their mechanical properties. Terrestrial olivines subjected to nanoindentation tests exhibit increased hardness and a higher Young's modulus than their lunar counterparts. A linear correlation is observed between the H/Er ratio and both plastic and elastic energies. Additionally, the alignment of mineral phases along a constant H/Er ratio suggests variations in local porosity. This study also highlights the need for further research focusing on porosity, phase insertions within the matrix, and structural orientations to refine our understanding of these mechanical characteristics. The findings have direct implications for in situ resource utilization strategies and future state‐of‐the‐art impact models. This comprehensive characterization serves as a foundational resource for future research efforts in space science and mining.

Reasonable value range of damage stress during rock brittle failure under compression

It is significant to clarify the reasonable value range of damage stress in the brittle failure process of rock to predict rock instability, such as collapse and landslide. Previous studies on damage stress were mostly limited to the statistical analysis of results obtained through laboratory rock mechanics experiments, and relevant theoretical studies are urgently needed. This study first derived the theoretical expression of the ratio of the rock damage stress to the peak stress σcd/σf by combining the three dimensional renormalization group model established by the proper stress transfer mechanism with the rock damage constitutive model, and obtained its theoretical range. Then, the rationality of the theoretical results was tested by analyzing the statistical results obtained through laboratory rock mechanics experiments for samples with different lithologies under different confining pressures. The reasonable value range of σcd/σf for describing the brittle failure of rock under compression is [0.76, 0.93]. The reasons for some experimentally obtained σcd/σf values falling outside of the reasonable range are discussed. The findings of this study can be useful for the investigation of disaster mechanisms and the development of rock mass instability prediction models.

Identification of different lithofacies laminations in oil shale and their mechanical properties

Lamination can greatly enhance the anisotropy and heterogeneity of shale and plays a significant role in influencing hydraulic fracturing. The structure and mechanical difference of different lithofacies lamination are the basis to reveal the fracture propagation mechanism. In this paper, research focused on three continental oil shale with different lithofacies in Qikou sag, Bohai Bay basin, including Quartz-Feldspar dominated shale, Carbonate dominated shale and mixed-mineral shale. Scanning electron microscopy and mineral analysis are used to identify quartz-feldspar lamination and mixture lamination in Quartz-Feldspar dominated shales, as well as carbonate lamination and mixture lamination in Carbonate dominated shales. The distinct laminations with different characteristics were precisely located using FIB, which served as the guiding tool for the indentation experiment. The micromechanical properties of laminations with different lithofacies in oil shale samples are examined using the nanoindentation technique, highlighting their distinct differences. The findings demonstrate that the micromechanical properties of quartz-feldspar lamination in Quartz-Feldspar dominated shales exhibit superior strength, while the mechanical difference between laminations in Quartz-Feldspar dominated shales are significantly larger. The quartz-feldspar lamination exhibits the highest resistance when subjected to indentation force. The mechanical properties of mixture lamination in Quartz-Feldspar dominated shales and Carbonate dominated shales are comparable due to their similar mineral composition. Moreover, in conjunction with macroscopic rock mechanics experiments, it has been verified that the lamination structure facilitates the initiation and propagation of macroscopic fractures under stress loading.

Dynamic behaviors and bolt-strengthening effect of sand-powder 3D printing rock analog under static and dynamic coupled loading

In deep mining, surrounding rock mass are subjected to high in-situ stress and dynamic loading, which leads to the difficulties in maintaining the roadway stability. Therefore, it is important to investigate the mechanical characteristics of rock bolts and their ability to enhance stability when exposed to static and dynamic coupled loading (SDCL). 3D printing technology can effectively address the challenges of preparing weak rock specimens and overcoming high variability. Consequently, the Split Hopkinson pressure bar (SHPB) is used to conduct SDCL testing on 3D printing specimens with varying bolted lengths. The similarity in mechanical performance between sand-powder 3D printed specimens and natural coal specimens has been validated. The rock dynamic characteristics with different bolted lengths under SDCL of specimens were explored. The macroscopic fragmentation and microscopic electron microscope scanning were used to reveal the dynamic failure characteristics of specimens. The study shows that static, dynamic and combined strength are positively correlated with bolted length. The dynamic strength is negatively correlated with the axial stress, and the combined strength initially increases and then subsequently decreases as the axial stress increases. The dynamic strength and the combined strength are positively correlated with the impact pressure. Efficient prevention of specimen fracture was achieved by increasing the length of the bolted connection. It can be found that at the meso-scale, when the specimen is only under the static loads, the cross-section particles are fractured along particle bonding, and under the SDCL, there are fractured along particle bonding and fractured through particle. The conclusions obtained can offer a foundation for conducting dynamic rock mechanics experiments using sand-powder 3D printed specimens and for controlling the surrounding rock in dynamic pressure roadways.

Evaluation of the remaining reserves of shale gas and countermeasures to increase the utilization of reserves: Case study of the Wufeng–Longmaxi formations in Changning area, southern Sichuan Basin, China

The evaluation of remaining reserves is crucial for assessing the developmental effect and further enhancing the recovery of a gas field. In this research, with the Changning shale gas field in the southern Sichuan Basin as the center of study, a comprehensive analysis was conducted on reservoir distribution, remaining reserves, and strategies to enhance recovery through the utilization of diverse methodologies, including organic geochemical testing, triaxial rock mechanics experiments, and numerical simulations. The results show that, in the study area, the recovery percentage of the well-controlled reserves ranges from 45% to 70%, with the average remaining reserves of wells falling within the (50–150) × 106 m3 range, alongside the potential for additional development in specific local areas. The Changning shale gas field exhibits three distinct types of undeveloped reserves, identified in areas where no wells have been drilled, inadequately fractured zones, and vertically undeveloped areas, respectively. In the areas where the average remaining reserves of wells are exceeding 100 × 106 m3, wells for repeated fracturing are selected depending on the coupling of geological, engineering, and development. In the case of well infilling, areas characterized by developed reticular fractures and existing well spacing >500 m are prioritized, taking into account the surface wellsite conditions. Through an extensive analysis, which include reservoir assessments, rock mechanics evaluations, and numerical modeling, sublayer⑤ is identified as the optimal target in the upper gas interval, with a vertical distance of more than 20 m from sublayer① in the lower gas interval. Zones with well-developed reticular natural fractures, a pressure coefficient >1.2, and a continuous thickness of Class I reservoirs in the upper gas interval >10 m, are selected for staggered tridimensional development with an expected increase in the platform-level recovery percent by 30%. These findings can provide valuable references and guidance for the deployment of well patterns in shale gas blocks.

Research on the Suitability of High-Density Leakage Plugging in Offshore High Temperature and High Pressure Wells

A large number of fractured formations are developed in the high-temperature and high-pressure formations of the Yingqiong Basin in the South China Sea. During drilling, the risk of lost circulation is ex-tremely high, which will cause complex downhole accidents and seri-ously affect the drilling speed.In order to realize safe and efficient plugging of HPHT Wells, the following studies are carried out in this paper: Firstly, the characteristics of leakage, types of leakage and plug-ging and prevention mechanism of HPHT formation in Yingqiong Ba-sin are clarified based on core and indoor rock mechanics experiments. Secondly, a simulated plugging evaluation device with high tempera-ture resistance of 180 °C and high pressure resistance of 20Mpa was in-vented and applied to accurately evaluate the reservoir leakage behav-ior in Yingqiong Basin. Finally, a series of anti-high temperature plug-ging agents adapted to different loss types are systematically selected for the first time for different formation fracture types and the plugging effect is evaluated. In this study, through mechanism research, device research and development, accurate evaluation, the shaft wall pressure equivalent increased to 0.1g/cm3 without leakage. It provides technical support for the subsequent ultra-high temperature and high pressure stratum with weak leakage and narrow pressure window.

Measurements of elastic properties and their dependencies within a damage mechanics workflow

Experimental rock mechanics testing provides a controlled and effective method for measuring physical properties, their dependencies, and their evolution due to the addition of localized microcracks. To understand the contributions of microcracks to first order changes in compliance, the behavior of initial undamaged properties of a material should be comprehensively investigated as a function of stress, load path, and load history. We perform a comprehensive study of elastic properties and their dependence on a variety of materials exhibiting nonlinearity, and varying levels of anisotropy in elastic stiffnesses. We programmatically perturb the testing environment of the specimens under triaxial stresses. Elastic moduli are measured within each test, and along multiple discrete loading paths for multistage tests as a function of stress, focusing on a set launch point. Four single stage triaxial tests per rock type are performed to calculate Mohr-Coulomb failure criteria, and ultrasonic velocities are captured during compression for establishing the upper bound of elastic behavior. Shear wave velocity for granite experiences a maximum value at a lower differential stress than maximum volumetric strain. Sandstone displays a similar trend at the highest confining pressure, while these two maxima converge under the lowest confining pressure.

Prediction of formation pressure based on numerical simulation of in-situ stress field: a case study of the Longmaxi formation shale in the Nanchuan area, eastern Chongqing, China

The in-situ stress and formation pressure are important parameters in shale gas development. They directly affect the well wall stability, the direction of horizontal well drilling, and the fracturing effect during the shale gas development process. There are abundant shale gas resources in the southeastern Chongqing-Sichuan area, but the structure in the area is complex, and it is difficult to predict the in-situ stress and formation pressure. Therefore, in this paper, a finite element simulation model was established based on a large number of seismic, logging, and experimental rock mechanics data and the prediction accuracy of the stress field simulation was effectively improved. The construction of the stress field was based on the combined spring model, as well as the data related to the measured in-situ stress and the formation pressure obtained during drilling. The coupling relationship between the in-situ stress, the formation strain, and the formation pressure were derived to carry out the prediction of the distributions of the formation pressure and the formation pressure coefficient. The prediction results showed that the present-day maximum principal stress direction in the study area was about NE65°–110°, and the present-day maximum principal stress was 56.12–93.79 MPa. The present-day minimum principal stress direction was about NE335°–20°, and the present-day minimum principal stress was 48.06–71.67 MPa. The formation pressure was 2.8–88.25 MPa, and the formation pressure coefficient was 0.74–1.55. The formation pressure distribution was greatly affected by fault, tectonic location, in-situ stress and rock petrophysical properties, and the overpressure areas of the formation were distributed in the synclines and the deeply buried areas. This study shows that the finite element based formation pressure prediction method is effective.

A comparison study on the predictive ability of numerical methods for fracturing of rock with different pre-existing flaws

Various numerical methods have been developed for modelling rock fracturing problems. Most of the available studies could reproduce reasonable results for given experimental data, but the ability and accuracy of these methods to predict rock fracturing are still controversial, and a systematic comparison is lacking. In this study, a set of rock mechanics experiments were designed for a numerical modelling contest to test the accuracy of different numerical methods in predicting rock fracturing. Six groups of specially designed experiments on rock fracturing were conducted on three types of rock specimens with pre-existing flaws. The first step was called the white box stage, in which the boundary conditions and experimental results (loading curve and fracture pattern) of two groups of experiments together with classical rock material parameters were provided to the participants who were asked to reproduce the experimental results (strength and fracture pattern) while allowing numerical parameter calibration. In the second stage, called the black box stage, the participants were asked to reproduce the remaining four groups of experiments with their corresponding calibrated numerical parameters. To ensure the solvability of these experimental data, we successfully reproduced the experimental observations and results in both white box and black box stages by using a numerical method with the same set of material parameters. We received complete numerical reports from ten teams using different numerical methods and found that most of the teams were able to accurately reproduce the experimental results in the white box stage but not the black box stage. The discontinuum-based methods with fewer input parameters, clear physical meaning, and easy adjustment provided better performance than the continuum-based methods. The experimental data presented in this study could provide a baseline for further improvement of the predictive ability of numerical methods for more accurate modelling of rock fracturing.

Stability evaluation of fault in hydrocarbon reservoir-based underground gas storage: A case study of W gas storage

Fault stability evaluation is crucial for ensuring safety of underground gas storage (UGS) reconstructed from depleted oil and gas reservoirs. To make assessment result more reasonable, a novel fault stability evaluation method is proposed, and four major faults in W gas storage are studied as a case to detailly describe this method. This method considers both shear rupture failure and frictional slip failure of fault based on Mohr-Coulomb criterion and Amonton’s law. In addition, dynamic variation in rock shear strength parameters under cyclic stress is understood and taken into account in fault stability evaluation by rock mechanics experiments. Meanwhile, specificity of fault friction coefficient in different UGSs is also considered in this method, and fault friction coefficient of W gas storage is determined by an empirical model. Fault stability evaluation results are quite disparate when different fault failure types are considered. For W gas storage, max operation pressure leading to fault failure is 62.10 MPa when shear rupture is regarded as fault failure type, and it reduce to 35.91 MPa when frictional slip is considered as fault failure type. Therefore, it is necessary to consider both two failure types, and further determine the key failure type in fault stability evaluation. Rock shear strength parameters vary with stress cycle, but the variation range is not significant in W gas storage. Rock cohesion shows a tendency that it fast declines first with stress cycles, then slow declines, and eventually tends to a constant. And variation in internal friction angle is contrary. Above both parameters orderly decrease by 3.08% and 8.75% after 53 cyclic stress cycles. The specificity of fault friction coefficient cannot be neglected in different UGSs. Otherwise, it could result in significant misestimate on fault stability of UGS. For W gas storage, fault friction coefficient is determined of 0.71 by empirical model, which is significant disparate to previous works. This work proposes an effective method which can help scholars and engineers to accurately evaluate fault stability in UGS.

A comprehensive feasibility evaluation of salt cavern oil energy storage system in China

Large-scale underground oil storage has a great effect on national energy safety. China's oil dependency has exceeded 70% for four consecutive years, so it is necessary for China to build strategic oil storage. The salt cavern is a good medium to store oil, and it is widely used in foreign countries. China is rich in salt mines and has basic conditions for building salt cavern oil storage (SCOS). In this paper, the necessity and advantages of SCOS are analyzed, the salt rock mechanics experiments of SCOS are conducted, the geological feasibility evaluation model is built, the 3D numerical creep stability model in three working conditions (long-term, oil pressure dynamic, injection and production) is built, and the economic feasibility evaluation model is built. The results show that the implementation of SCOS in China is feasible, the earthquake (51%) and fault (28%) have a great effect on geological selection, the SCOS has good stability under three working conditions, the storage cost of SCOS is lower by 64.3% and 84.3% contrasted to rock cavern and storage tank in 2025. The study has a reference value for the development of SCOS in China.